I do not believe there should be an opt-out feature for responsive themes.

The purpose of the site being responsive is to improve the experience across devices. If the user feels frustrated or feels like something is missing and wants to switch out of the responsive version of a site, then I would argue that the designer has not done their job well.

For example: if the user feels that a feature is missing from the responsive version, the designer shouldn’t have removed it. If the user wants to pinch or zoom in on text, then the designer hasn’t sized the typography properly.

I do understand what you are saying though. My belief is that designing better responsive sites is the answer. If the user wants to opt-out then the designer has failed to create a successful responsive design.

Don’t get me wrong though. Just a couple years ago when we had separate mobile sites the experiences were for the most part horrible. I almost always wanted to switch to the full site. Responsive designs today however, I find myself appreciating for the most part.

Some designers are lazy when it comes to responsive design. They quickly adjust the desktop version of their site to fit an iPad or iPhone and think they are finished, just to add it as a “feature”. Good responsive design requires careful planning and should not be an afterthought.